1. Field of the Invention
This invention relates to brazing of metal parts; and more particularly, to a homogeneous, ductile iron-chromium-based brazing material useful in brazing stainless steels, and a method for brazing stainless steel components to form articles of manufacture that reduces the propensity of nickel to leach from such articles in water.
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, leak-tight joint is formed. The assembled parts may either constitute a finished article of manufacture or they may form a sub-component for use in a further manufacturing operation.
The selection of a particular brazing filler metal for a specific application depends on a variety of factors, including requirements related to the components to be joined and to the conditions under which the assembly ultimately must operate.
One basic consideration is temperature. Brazing filler metals are characterized by their solidus and liquidus temperatures. The term “solidus” refers to the highest temperature at which a metal or alloy is completely solid, and the term “liquidus” refers to the lowest temperature at which the metal or alloy is completely liquid. In any brazing process, the brazing filler metal must possess a solidus temperature that is high enough to provide the brazed assembly with adequate integrity to meet the desired service requirements and yet have a liquidus that is low enough to be compatible with the temperature capabilities of the parts being joined.
Another consideration is corrosion resistance. Many brazed assemblies must operate under environmental conditions that are conducive to corrosion, especially in the vicinity of the brazement. The propensity of a given system to corrode is strongly influenced by the gases or liquids to which the system is exposed and by typical operating temperatures.
One class of devices which are frequently assembled using brazing as a joining technique is heat exchangers. These devices are known in a variety of configurations. Generally stated, heat exchangers allow heat to be transferred across an interface that separates one circulating fluid from another circulating fluid. It is generally essential that the fluids, either of which can be gaseous or liquid, be kept separate. Hence, it is critical that brazed joints which define, at least in part, the interface maintain structural integrity under a full range of operating conditions and for a prolonged service life.
One field of use wherein heat exchangers find utility is in the processing of materials which are ultimately intended for human ingestion and consumption. These include foodstuffs, as well as fluids such as water, beverages, juices, and the like. The metallic materials used for the construction of heat exchangers appointed for such applications are of critical importance. They not only need to provide excellent operative characteristics with regard to heat transfer, but also must be compatible with the substances to which they are exposed. One particular concern is the requirement that there be no undesired leaching or elution of any elemental or molecular component species of the materials of construction that is harmful or adds undesirable taste to the fluids. If a harmful species or an undesirable taste is present, then it is imperative that any leaching of causative materials be minimized. Frequently, local governmental or regulatory authorities have established maximum amounts of materials, such as metal ions, which may be permitted to leach into fluids passing therethrough. The standard is ordinarily expressed as a maximum amount of leachate that may be present per unit volume of the fluid processed. Ideally, the materials incorporated in heat exchangers (including brazing filler metals) and the associated manufacturing methods result in a device that meets or exceeds applicable regulatory standards under foreseeable operating conditions.
Heat exchangers of the “shell-and-tube,” “plate/plate,” and “plate/fin” types are most usually encountered. In the first configuration, a larger diameter housing typically referred to as a “shell” encompasses one or more small diameter tubes or pipes. According to this configuration, a first fluid (i.e., liquid, gas) passes through the shell and about the exterior of the tubes while simultaneously, a second fluid (liquid, gas) passes through the interior of the tubes. While no physical contact is permitted between the first and second fluids, heat transfer occurs across the walls of the tubes from the hotter fluid to the cooler fluid. In plate/plate and plate/fin type heat exchangers, again a physical member, namely one or more plates separate a first fluid from a second fluid while heat transfer occurs across the plate. In these types of heat exchanger (as well as in other assemblies), metals are most commonly used due to their high strength and good heat transfer characteristics. Typically, the individual parts, which are used to make up these types of heat exchangers, are joined by brazing. It is imperative that the heat exchanger maintain its physical integrity and the isolation of the fluids from each other and the outside world. In addition, the heat exchanger and the joints that secure its internal components must be resistant to any potential detrimental effects which might result from contact with one or both of the fluids.
To minimize this undesired technical effect, the materials of construction for heat exchangers, particularly those used for foodstuffs, need to be very carefully selected. Stainless steels, which contain up to about 20% Ni, are very commonly encountered, for they exhibit desirable properties including low leaching rates into fluids or gases, and generally good corrosion resistance. However, brazing manufacturing processes carried out at high temperatures may also adversely affect the propensity of the stainless steels to leach. Previously, elemental copper was used as a brazing filler metal as such featured low leaching of nickel into fluids, especially water. However, the corrosion resistance of heat exchangers having components brazed using copper as brazing filler metal is poor. Typically these heat exchangers required frequent replacement, resulting in significant costs for the replacement device and the associated labor, as well as economic losses resulting from manufacturing downtime. To improve corrosion resistance, it was recently found that brazing filler metals with compositions based primarily on nickel and chromium (“Ni/Cr”) could be employed to join stainless steel parts used in such assemblies. Unfortunately, it was also found that when such Ni/Cr-based brazing filler metals were used, an undesirably high amount of nickel often leached into water or other fluids flowing through these assemblies.
Inasmuch as such Ni/Cr-based brazing filler metals include a significant proportion of nickel, they are believed to be the source of the undesired nickel leachate. For this reason, use of Ni/Cr-based brazing filler metals should be avoided in applications where nickel leaching into a fluid presents a concern, as is the case when materials are appointed to be used for human ingestion or consumption. Not surprisingly, governmental regulations in some countries have imposed strict limitations on the amount of nickel which may be leached into these fluids. It is to one or more of these technical needs that the present invention is directed.